Method for producing methacrylic acid

ABSTRACT

In a method for synthesizing methacrylic acid by using a fixed bed tube type reactor provided with a catalyst bed into which a solid oxidation catalyst is filled and with a heat medium bath and by flowing a raw material gas containing methacrolein and oxygen through the catalyst bed, the catalyst bed does not have any sections in which a temperature difference between the heat medium bath and the catalyst bed (ΔT) exceeds 35° C., and two or more high temperature zones in which each ΔT is 15 to 35° C. are provided. According to this method, mathacrylic acid can be manufactured in higher yields.

TECHNICAL FIELD

This invention relates to a method for manufacturing methacrylic acid byvapor phase catalytic oxidation of methacrolein with molecular oxygen inthe presence of a solid oxidation catalyst using a fixed bed tube typereactor.

BACKGROUND ART

There have been many suggestions of catalysts which are used inmanufacturing methacrylic acid by a vapor phase catalytic oxidationreaction of methacrolein (this vapor phase catalytic oxidation reactionwill be referred to merely as “oxidation reaction,” unless otherwisespecified hereinafter). These suggestions are mainly related to elementsconstituting the catalysts and a ratio of the elements.

The oxidation reaction causes heat accumulation in a catalyst bedbecause this reaction is an exothermic reaction. Locally abnormal hightemperature zones resulting from excessive heat accumulation are knownas hot spots, in which yields are reduced by the excessive oxidationreaction. Thus, when conducting the oxidation reaction on an industrialscale, occurrence of the hot spots is a significant problem, andparticularly in the case where a concentration of methacrolein in a rawmaterial gas is increased in order to increase productivity, the hotspots tend to easily occur, so that the reaction conditions are requiredto be largely restricted under the present circumstances.

Therefore, it is very important to control the temperature at the hotspot section for producing methacrylic acid in high yield on anindustrial scale. In addition, especially in case of using a molybdenumcontaining solid oxidation catalyst, it is important to prevent theoccurrence of the hot spots because the molybdenum moiety is easy to besublimated.

Several suggestions have been made as for a method for controlling thetemperature at the hot spot section. For example, Japanese PatentApplication Laid-Open No. 4-210937 discloses a method in which aplurality of catalysts having different activities are filled in such amanner that their activities become gradually higher from an inlet sidefor a raw material gas toward an outlet side, then the raw material gascontaining methaclorein and oxygen is flown through this catalyst bed.Japanese Patent Application Laid-Open No. 8-92154 discloses a method inwhich, when acrolein is subjected to vapor phase oxidation using amulti-tube type fixed bed reactor provided with a heat medium bath toproduce acrylic acid, a flow of the heat medium is controlled so thatthe temperature of the heat medium bath is raised by 2 to 10° C. betweenthe inlet section and the outlet section.

However, these methods are only for the purpose of decreasing thetemperature of the hot spot section, and are the methods which merelyreduce a temperature difference (ΔT) between the heat medium bath andthe catalyst bed in one hot spot section to some extent. That is,according to these methods, heat generation caused by oxidation reactionhas not been actively controlled within the catalyst bed, so that, forexample when a methacrolein concentration is further increased in orderto increase productivity, heat generation at the hot spot sectionbecomes remarkable. Hence, the reaction conditions are still required tobe largely restricted. That is, under the present circumstances, theyield of methacrylic acid which is industrially acceptable has not beenobtained when a concentration of methacrolein is increased to a levelwhich is industrially acceptable.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide a method formanufacturing methacrylic acid by vapor phase catalytic oxidation ofmethacrolein with molecular oxygen in the presence of a solid oxidationcatalyst in a fixed bed tube type reactor, wherein metharylic acid ismanufactured in a higher yield.

The present invention is a method for manufacturing methacrylic acidcharacterized in that methacrylic acid is synthesized by filling a solidoxidation catalyst into a fixed bed tube type reactor provided with aheat medium bath and by flowing a raw material gas containingmethacrolein and oxygen through the catalyst bed and the catalyst beddoes not have any sections in which a temperature difference between theheat medium bath and the catalyst bed (ΔT=catalyst bed temperature −heatmedium bath temperature) exceeds 35° C. and two or more high temperaturezones in which each ΔT thereof is 15 to 35° C. are provided.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, a reaction for synthesizingmethacrylic acid is carried out using a fixed bed tube type reactorprovided with a heat medium bath. Although there are no limitations onthe heat medium used herein, molten salts containing potassium nitrateand sodium nitrite are typically employed. There are also no limitationson the tube type reactor, but on an industrial scale, it is preferableto use a multi-tube type reactor provided with several thousands toseveral ten thousands of reaction tubes each of which has an innerdiameter of 20 to 30 mm and each of which is surrounded by a heat mediumbath.

In the present invention, the solid oxidation catalyst used herein isnot particularly restricted as long as the catalyst is a solid catalystfor use in this oxidation reaction. Phosphorus and molybdenum containingcomposite oxides or the like which are previously known can be used, andthe composite oxides represented by the following formula (1) arepreferred:Mo_(a)P_(b)Cu_(c)V_(d)X_(e)Y_(f)O_(g)  (1)In the formula (1), Mo, P, Cu, V and O represent molybdenum, phosphorus,copper, vanadium, and oxygen, respectively, X represents at least oneelement selected from a group consisting of iron, cobalt, nickel, zinc,magnesium, calcium, strontium, barium, titanium, chromium, tungsten,manganese, silver, boron, silicon, tin, lead, arsenic, antimony,bismuth, niobium, tantalum, zirconium, indium, sulfur, selenium,tellurium, lanthanum, and cerium, and Y represents at least one elementselected from a group consisting of potassium, rubidium, cesium, andthallium. In the above formula (1), a, b, c, d, e, f, and g representatomic ratios of the respective elements. When a=12, 0.1≦b≦3, 0.01≦c≦3,0.01≦d≦3, 0≦e≦3, 0.01≦f≦3, and g is an atomic ratio of oxygen which isrequired to satisfy a valence of the above described each component.Particularly preferable atomic ratios of the respective elements are0.2≦b≦2, 0.04≦c≦1, 0.1≦d≦2, 0≦e≦2, and 0≦f≦2, when a=12.

A method for preparing a catalyst used in the present invention is notparticularly limited, and the previously well-known various methods canbe used unless the components thereof extremely unevenly distributedwithin a catalyst.

Raw materials used for preparing the catalyst are not particularlylimited, and nitrates, carbonates, acetates, ammonium salts, oxides,halides or the like of each element can be employed in combination. Forexample, ammonium paramolybdate, molybdenum trioxide, molybdic acid,molybdenum chloride or the like can be used as a raw material formolybdenum.

Although a catalyst used in the present invention may be carrier-free,it is possible to use a catalyst supported on an inactive carrier suchas silica, alumina, silica-alumina, or silicon carbide, or a catalystdiluted by these carriers.

In the present invention, a catalyst bed means a space sectioncontaining at least a catalyst within a reaction tube of a fixed bedtube type reactor. That is, a catalyst bed is not only a space which isfilled only with a catalyst but also a space section in which thecatalyst is diluted with the inactive carrier or the like. However,space sections at both ends of the reaction tube into which no substanceis filled or space sections into which only inactive carriers or thelike are filled are not included to the catalyst bed, because thesesections substantially do not contain any catalysts.

In the present invention, when synthesizing methacrylic acid by fillinga solid oxidation catalyst into a fixed bed tube type reactor providedwith a heat medium bath and by flowing a raw material gas containingmethacrolein and oxygen through the catalyst bed, it is important toprovide two or more high temperature zones within the catalyst bed inwhich the temperature difference between the heat medium bath and thecatalyst bed (ΔT) is 15 to 35° C. A smaller maximum value of ΔT in thecatalyst bed is preferred, and the maximum value ΔT should be not largerthan 35° C. because if a value of ΔT becomes too larger, an excessiveoxidation reaction causes a reduction in selectivity, which leads to areduction of the yields, and there is also a possibility of degradationand alteration of the catalyst property due to thermal load. Anoxidation reaction in the present invention is an exothermic reaction,so that it is inevitable that a ΔT which is a certain degree ofmagnitude is produced. However, controlling the conditions in such amanner that two or more high temperature zones in which each ΔT is 15°C. or more are provided, it is possible to avoid producing a locallyabnormal high temperature zone concentrated in one region. As a matterof course, zones each ΔT of which is less than 15° C. should exist amongthese respective high temperature zones.

Methods for controlling the conditions in such a manner that two or morehigh temperature zones are provided are not particularly limited andinclude, for example, such as a method in which catalysts are filledinto respective reaction zones provided by dividing a space within areaction tube into two or more regions in its tube axial direction and alength of each reaction zone in its tube axial direction is adjusted andthe catalytic activity per unit volume in each reaction zone iscontrolled. In this case, the methods for controlling the catalyticactivity per unit volume include, for example, such as a method forcontrolling a dilution ratio when the catalyst is diluted with aninactive carrier, and a method for controlling by using other catalystshaving different activities which are obtained by modifying a catalystcomposition or a preparing process thereof.

It is preferable that the two or more high temperature zones in whicheach ΔT is within a range of 15 to 35° C. are provided in a catalyst bedin such a manner that a distance between a first high temperature zoneand a second high temperature zone from an inlet side for the rawmaterial gas is 0.2 to 0.9 times the overall length of the catalyst bed,and particularly, it is preferable that the distance should be 0.25 to0.8 times the overall length of the catalyst bed. A proportion of thecatalyst effectively acting in the whole catalyst filled into thereactor tends to increase as the ratio of the distance between the hightemperature zones to the overall length of the catalyst bed becomessmaller, whereas an yield of methacrylic acid tends to be higher and thepossibility resulting in the degradation and alteration of the catalysttends to be reduced as the above described ratio becomes larger. Whenthere are three or more high temperature zones, the distance between thetwo adjacent high temperature zones may be 0.2 to 0.9 times the overalllength of the catalyst bed, and particularly 0.25 to 0.8 times theoverall length thereof. The number of the high temperature zones istypically five or less, and practically two or three are preferable.

And the distance between the high temperature zones represents adistance between the points at each of which the ΔT is maximum in thehigh temperature zone.

A position of the first high temperature zone (a position wherein the ΔTis maximum within this high temperature zone) is preferable at adistance of 0 to 0.7 times the overall length of the catalyst bed fromthe inlet for the raw material gas, and it is particularly preferable ata distance of 0.1 to 0.5 times the overall length of the catalyst bedfrom the above described inlet.

Usually, minute peaks and valleys can be seen on the ΔT curve which isin a tube axial direction, because of slightly nonuniform distributionof the filled catalyst. Hence, when determining a range of the hightemperature zone, noise caused by the peaks and valleys is reduced bydetermining a mean ΔT for the measured ΔTs around the certainmeasurement position within a range of 0.005 times, preferably 0.01times, the overall length of the catalyst bed. Even if an exothermicpeak which satisfies a requirement of the high temperature zone isobserved at the measured ΔT, in the case where the requirement of thehigh temperature zone is not satisfied when based on the mean ΔT, theexothermic peak should not be taken as the high temperature zone.

In the present invention, ΔT in the catalyst bed means a differencebetween temperature at a certain measurement position within thecatalyst bed and temperature of a heat medium bath around the position.Although there is a possibility of nonuniform distribution of heatmedium temperatures within the heat medium bath depending on, such as, areactor configuration, reaction conditions, and a flowing state of theheat medium, there is no problem in treating the mean temperature of theheat medium bath as a heat medium bath temperature in the case where adegree of the nonuniform distribution is small. However, in the casewhere the degree is not small, it is required to determine ΔT bymeasuring the heat medium temperature at each position.

Further, methods for measuring temperatures in the catalyst bed include,for example, a method in which, prior to filling a catalyst, protectivetubes are placed in the tube type reactor and a thermocouple is insertedinto the respective protective tubes to measure the temperatures at therespective positions during reaction. In this method, a position toplace the protective tube is preferably a center of a section which isnormal to the tube axial direction of the reaction tube, and a length ofthe protective tube is required to be longer than that of the catalystbed. This method is preferable because the temperatures at any positionswithin the catalyst bed can be conveniently measured. In addition, incase of using a multi-tube type reactor which is industrially employed,it is actually impossible to measure the temperatures of the catalystbeds of all reaction tubes, so that some of the reaction tubes whichrepresent the entire reactor will be actually subjected to thismeasurement.

In practice of the present invention, a concentration of methacrolein ina raw material gas can be varied within a wide range, but it isappropriate to be in a range of 1 to 20 volume %, and particularlypreferable to be in a range of 3 to 8 volume %.

It is economically advantageous to use air as a source of oxygen, butair enriched with pure oxygen may be used if necessary. A concentrationof oxygen in the raw material gas is preferably 0.3 to 4 moles relativeto one mole of methacrolein, particularly it is preferable to be 0.4 to3 moles. The raw material gas may contain a little amount of impuritiessuch as lower saturated hydrocarbyl aldehyde which do not substantiallyinfluence on a primary reaction and may also be diluted by addinginactive gases such as nitrogen, steam, and carbon dioxide.

A reaction pressure for oxidation reaction can be varied from anatmospheric pressure to several atmospheres. The heat medium bathtemperature which is a reaction temperature is preferable 230 to 450°C., and particularly preferable 250 to 400° C. A space velocity of a rawmaterial gas is preferable 300 to 3000 hr⁻¹, and particularly preferable500 to 2000 hr⁻¹.

EXAMPLES

The present invention will now be further described in detail byexamples. In the examples and comparative examples, “part(s)” meanspart(s) by weight. A composition of a catalyst was determined from thecharged amounts of raw materials of catalyst components. As a heatmedium of a reactor, a molten salt composed of 50 mass % of potassiumnitrate and 50 mass % of sodium nitrite was used. An analysis ofreaction raw materials and products was carried out by gaschromatography.

In addition, a conversion of methacrolein, a selectivity of producedmethacrylic acid, and a one-pass yield of methacrylic acid arerespectively defined as follows.Conversion of methacrolein (%)=(B/A)×100Selectivity of methacrylic acid (%)=(C/B)×100One-pass yield of methacrylic acid (%)=(C/A)×100In the above definitions, A is the number of moles of suppliedmethacrolein, B is the number of moles of reacted methacrolein, and C isthe number of moles of produced methacrylic acid.

Example 1

100 parts of ammonium paramolybdate, 2.8 parts of ammonium metavanadate,and 9.2 parts of cesium nitrate were dissolved into 300 parts of purewater. A solution of 8.2 parts of 85 mass % phosphoric acid in 10 partsof pure water and a solution of 1.1 parts of telluric acid in 10 partsof pure water were added to above solution with stirring, andtemperature was raised to 95° C. with stirring. Then, a solution of 3.4parts of copper nitrate, 7.6 parts of ferric nitrate, 1.4 parts of zincnitrate, and 1.8 parts of magnesium nitrate in 80 parts of pure waterwas added. Further, this mixture was stirred for 15 minutes at 100° C.,and an obtained slurry was dried using a spray dryer.

Two parts of graphite were added to 100 parts of the obtained driedsubstance and mixed with each other, then pressed into ring tabletswhich were 5 mm in outer diameter, 2 mm in inner diameter, and 3 mm inlength by using a tabletting machine. The tablets were calcined at 380°C. for 5 hours while flowing air, then a catalyst 1 was obtained. Thecomposition of the catalyst 1, represented by an atomic ratio excludingoxygen, was as follows:Mo₁₂P_(1.5)Cu_(0.3)V_(0.5)Fe_(0.4)Te_(0.1)Mg_(0.15)Zn_(0.1)Cs₁

A mixture of 370 mL of the catalyst 1 and 130 mL of alumina balls havingan outer diameter of 5 mm was filled into an inlet part for a rawmaterial gas of a steel made fixed bed tube type reactor having an innerdiameter of 25.4 mm which was provided with a heat medium bath, and 1000mL of the catalyst 1 was filled into an outlet part of the abovedescribed reactor. In this case, a length of the catalyst bed was 3005mm. A raw material gas consisting of 6.5 volume % of methacrolein, 11volume % of oxygen, 10 volume % of steam, and 72.5 volume % of nitrogenwas passed through this catalyst bed at a space velocity of 1000 hr⁻¹and at a reaction temperature (a heat medium bath temperature) of 290°C.

Upon measuring a temperature of the catalyst bed in this case, a firsthigh temperature zone in which a maximum temperature point was 300 mmfrom an end of the inlet side for the raw material gas and a second hightemperature zone in which a maximum temperature point was 1200 mm fromthe end of the inlet side for the raw material gas were observed. Thatis, a ratio of a distance between the above two high temperature zonesto the length of the catalyst bed was 0.30. In addition, ΔT at theposition of the maximum temperature in the first high temperature zonewas 21° C., and ΔT at the position of the maximum temperature in thesecond high temperature zone was 19° C. Further, ΔT at a position 1000mm from the end of the inlet side for the raw material gas was 12° C.

Table 1 shows analytical results for the reaction products which werecollected.

Comparative Example 1

A mixture of 620 mL of the catalyst 1 and 130 mL of alumina balls havingan outer diameter of 5 mm was filled into an inlet part for a rawmaterial gas of the same fixed bed tube type reactor as used in Example1, and 750 mL of the catalyst 1 was filled into an outlet part of thisreactor. A length of the catalyst bed in this case was 3005 mm. The sameraw material gas as used in Example 1 was passed through this catalystbed under the same conditions.

Then measuring the temperature of the catalyst bed, only one hightemperature zone was observed in which the maximum temperature point was400 mm from the end of the inlet side for the raw material gas. Inaddition, ΔT at the maximum temperature of this high temperature zonewas 31° C.

Table 1 shows results of analysis of the reaction product.

Comparative Example 2

1500 mL of the catalyst 1 was filled into the same reactor as used inExample 1. A length of the catalyst bed at this time was 3005 mm. Thesame raw material gas as used in Example 1 was passed through thiscatalyst bed under the same conditions.

Then measuring the temperature of the catalyst bed, only one hightemperature zone was observed in which the maximum temperature point was200 mm from the end of the inlet side of the raw material gas. Inaddition, ΔT at the maximum temperature of this high temperature zonewas 40° C.

Table 1 shows results of analysis of the reaction product.

Comparative Example 3

1370 mL of the catalyst 1 was filled into the same reactor as used inExample 1. A length of the catalyst bed at this time was 2745 mm. Thesame raw material gas as used in Example 1 was passed through thiscatalyst bed under the same conditions.

Then measuring the temperature of the catalyst bed, only one hightemperature zone was observed in which the maximum temperature point was200 mm from the end of the inlet side of the raw material gas. Inaddition, ΔT at the maximum temperature of this high temperature zonewas 40° C.

Table 1 shows results of analysis of the reaction product.

Comparative Example 4

A mixture of 220 mL of the catalyst 1 and 130 mL of alumina balls havingan outer diameter of 5 mm was filled into an inlet part for a rawmaterial gas of the same reactor as used in Example 1, and 1150 mL ofthe catalyst 1 was filled into an outlet part of this reactor. A lengthof the catalyst bed at this time was 3005 mm. The same raw material gasas used in Example 1 was passed through this catalyst bed under the sameconditions.

Upon measuring a temperature of the catalyst bed at this time, a firsthigh temperature zone in which the maximum temperature point was 250 mmfrom an end of the inlet side for the raw material gas and a second hightemperature zone in which the maximum temperature point was 830 mm fromthe end of the inlet side of the raw material gas were observed. Thatis, a ratio of a distance between the above two high temperature zonesto the length of the catalyst bed was 0.19. In addition, ΔT at themaximum temperature of the first high temperature zone was 16° C., andΔT at the maximum temperature of the second high temperature zone was37° C. Further, ΔT at a position 700 mm from the end of the inlet sideof the raw material gas was 10° C.

Table 1 shows results of analysis of the reaction product.

Example 2

100 parts of molybdenum trioxide, 3.2 parts of vanadium pentaoxide, and6.7 parts of 85 mass % phosphoric acid were mixed with 800 parts of purewater. After stirring this mixture for three hours while heating atreflux, 0.5 part of copper oxide, 0.7 part of boric acid, and 1.2 partsof germanium dioxide were added to the mixture, then the mixture wasstirred again for two hours while heating at reflux. The obtained slurrywas cooled to 50° C., and a solution of 11.2 parts of cesium bicarbonatein 30 parts of pure water was added to this slurry and stirred for 15minutes. Then, a solution of 10 parts of ammonium nitrate in 30 parts ofpure water was added to the slurry and further stirred for 15 minutes,and the obtained slurry containing catalyst components was dried using aspray dryer.

Two parts of graphite were added to 100 parts of the obtained driedsubstance and mixed with each other, then pressed into ring tabletswhich had an outer diameter of 5 mm, an inner diameter of 2 mm, and alength of 3 mm using a tabletting machine. The tablets were calcined at380° C. for 5 hours while flowing air, then a catalyst 2 was obtained.The composition of the catalyst 2 represented by an atomic ratio,exclusive of oxygen, wasMo₁₂P₁Cu_(0.1)V_(0.6)Ge_(0.2)B_(0.2)Cs₁

A mixture of 150 mL of the catalyst 2 and 90 mL of alumina balls havingan outer diameter of 5 mm was filled into an inlet part for a rawmaterial gas of the same fixed bed tube type reactor as used in Example1, and a mixture of 200 mL of the catalyst 2 and 40 mL of alumina ballshaving an outer diameter of 5 mm was filled into a middle part of thisreactor, and 1020 mL of the catalyst 2 was filled into an outlet part ofthis reactor. A length of the catalyst bed at this time was 3005 mm. Thesame raw material gas as used in Example 1 was passed through thiscatalyst bed under the same conditions.

Upon measuring a temperature of the catalyst bed, a first hightemperature zone in which the maximum temperature point was 180 mm froman end of the inlet side of the raw material gas, a second hightemperature zone in which the maximum temperature point was 620 mm fromthe end of the inlet side of the raw material gas, and a third hightemperature zone in which the maximum temperature point was 1100 mm fromthe end of the inlet side of the raw material gas were observed. Thatis, among the above three adjacent high temperature zones, a ratio of adistance between the first and the second high temperature zones to thelength of the catalyst bed was 0.15, and a ratio of a distance betweenthe second and the third high temperature zones to the length of thecatalyst bed was 0.16. In addition, ΔT at the maximum temperature of thefirst high temperature zone was 16° C., ΔT at the maximum temperature ofthe second high temperature zone was 18° C., and ΔT at the maximumtemperature of the third high temperature zone was 17° C. Further, ΔT ata position 480 mm from the end of the inlet side for the raw materialgas was 9° C., and ΔT at a position 960 mm from the end of the inletside for the raw material gas was 9° C.

Table 1 shows results of analysis of the reaction product.

TABLE 1 Examples, Conversion of Selectivity of Yield of Uni-flowComparative Methacrolein Methacrylic acid Methacrylic acid Examples (%)(%) (%) Example 1 84.5 84.0 71.0 Comparative 85.7 81.9 70.2 Example 1Comparative 87.9 78.1 68.6 Example 2 Comparative 87.3 78.3 68.4 Example3 Comparative 86.3 79.5 68.6 Example 4 Example 2 85.2 85.4 72.8

INDUSTRIAL APPLICABILITY

According to the present invention, when synthesizing methacrylic acidby filling a solid oxidation catalyst into a fixed bed tube type reactorprovided with a heat medium bath and by flowing a raw material gascontaining methacrolein and oxygen through the catalyst bed, methacrylicacid can be manufactured in high yields by providing two or more hightemperature zones in which each ΔT is 15 to 35° C. without providing anysections in which ΔTs exceed 35° C.

Also, according to the present invention, the yields are furtherincreased by providing a first high temperature zone and a second hightemperature zone from an inlet side for the raw material gas in such amanner that a distance between these high temperature zones is 0.2 to0.9 times an overall length of the catalyst bed.

Further, the yields are also increased by employing a composite oxiderepresented by the above described formula (1) as a solid oxidationcatalyst.

1. A method for manufacturing methacrylic acid, comprising: passing araw material gas containing methacrolein and oxygen through a catalystbed of a solid oxidation catalyst in a fixed bed tube type reactor thatis immersed in a heat medium bath, thereby oxidizing the methacrolein tomethacrylic acid, wherein the catalyst bed does not have any sections inwhich the temperature difference between the temperature of the heatmedium bath and the temperature of the catalyst bed {ΔT=(catalyst bedtemperature)−(heat medium bath temperature)} exceeds 35° C. but doeshave at least two high temperature zones wherein the temperature of eachof said zones falls within the temperature region of 15 to 35° C. abovethe temperature of the heat medium bath.
 2. The method for manufacturingmethacrylic acid according to claim 1, wherein the distance between afirst high temperature zone and a second high temperature zone withinthe solid oxidation catalyst bed of the reactor taken from the inlet ofthe reactor through which the raw material gas enters the reactor is 0.2to 0.9 times the overall length of the catalyst bed.
 3. The method formanufacturing methacrylic acid according to claim 1, wherein said solidoxidation catalyst is a composite oxide represented by formula (1):Mo_(a)P_(b)Cu_(c)V_(d)X_(e)Y_(f)O_(g)  (1) wherein Mo, P, Cu, V and Orepresent molybdenum, phosphorus, copper, vanadium, and oxygen,respectively, X represents at least one element selected from the groupconsisting of iron, cobalt, nickel, zinc, magnesium, calcium, strontium,barium, titanium, chromium, tungsten, manganese, silver, boron, silicon,tin, lead, arsenic, antimony, bismuth, niobium, tantalum, zirconium,indium, sulfur, selenium, tellurium, lanthanum, and cerium, and Yrepresents at least one element selected from the group consisting ofpotassium, rubidium, cesium, and thallium, wherein a, b, c, d, e, f andg represent atomic amounts of the respective elements, and when a=12,0.1≦b≦3, 0.01≦c≦3, 0.01≦d≦3, 0≦e≦3 and 0.01≦f≦3, g is the amount ofoxygen which is required to satisfy the summed valences of the Mo, P,Cu, V, X and Y components.
 4. The method for manufacturing methacrylicacid according to claim 2, wherein said solid oxidation catalyst is acomposite oxide represented by formula (1):Mo_(a)P_(b)Cu_(c)V_(d)X_(e)Y_(f)O_(g)  (1) wherein Mo, P, Cu, V and Orepresent molybdenum, phosphorus, copper, vanadium, and oxygen,respectively, X represents at least one element selected from the groupconsisting of iron, cobalt, nickel, zinc, magnesium, calcium, strontium,barium, titanium, chromium, tungsten, manganese, silver, boron, silicon,tin, lead, arsenic, antimony, bismuth, niobium, tantalum, zirconium,indium, sulfur, selenium, tellurium, lanthanum, and cerium, and Yrepresents at least one element selected from the group consisting ofpotassium, rubidium, cesium, and thallium, wherein a, b, c, d, e, f andg represent atomic amounts of the respective elements, and when a=12,0.1≦b≦3, 0.01≦c≦3, 0.01≦d≦3, 0≦e≦3 and 0.01≦f≦3, g is the amount ofoxygen which is required to satisfy the summed valences of the Mo, P,Cu, V, X and Y components.
 5. The method for manufacturing methacrylicacid according to claim 1, wherein said temperature difference ΔT andthe temperatures of the high temperature zones are controlled byadjusting the activity of the catalyst per unit volume within saidcatalyst bed.
 6. The method for manufacturing methacrylic acid accordingto claim 2, wherein said temperature difference ΔT and the temperaturesof the high temperature zones are controlled by adjusting the activityof the catalyst per unit volume within said catalyst bed.
 7. The methodfor manufacturing methacrylic acid according to claim 3, wherein saidtemperature difference ΔT and the temperatures of the high temperaturezones are controlled by adjusting the catalytic activity per unit volumewithin said catalyst bed.
 8. The method for manufacturing methacrylicacid according to claim 4, wherein said temperature difference ΔT andthe temperatures of the high temperature zones are controlled byadjusting a catalytic activity per unit volume within said catalyst bed.9. The method for manufacturing methacrylic acid according to claim 2,wherein the distance between a first high temperature zone and a secondhigh temperature zone within the solid oxidation catalyst bed of thereactor taken from the inlet of the reactor through which the rawmaterial gas enters the reactor is 0.25 to 0.8 times the overall lengthof the catalyst bed.
 10. The method for manufacturing methacrylic acidaccording to claim 1, wherein the position of the first high temperaturezone (ΔT is maximum) within the solid oxidation catalyst bed of thereactor taken from the inlet of the reactor is at a distance of 0 to 0.7times the overall length of the solid oxidation catalyst bed.
 11. Themethod for manufacturing methacrylic acid according to claim 10, whereinthe position of the first high temperature zone (ΔT is maximum) withinthe solid oxidation catalyst bed of the reactor taken from the inlet ofthe reactor is at a distance of 0.1 to 0.5 times the overall length ofthe solid oxidation catalyst bed.